PRB Articles

Jan 31 Native Vs. Nonnative

Today, perhaps more than ever before, there are good reasons for landscape architects and restoration ecologists to join forces in developing basic ecological and design principles that apply in all land-use decisions.

It's important for landscape architects to look at the ecological impact of their designs and the plants they use. Photo Courtesy Of Applied Ecological Services, Inc.

While we have come to our professions with different training—one heavily based on design principles, the other heavily based on scientific principles—our aims are often aligned.

A shared goal is to create landscapes with ecosystems that are low-maintenance yet aesthetically pleasing, and which provide beneficial services, such as aesthetics, noise abatement, stormwater control, or habitat for wild fauna, while also providing for human needs.

Much attention has been focused on the use of native vs. nonnative species in projects. However, too often this debate ignores an important underlying principle of restoration: The focus should be on restoring ecosystem functions rather than on the species.

More than 40 years ago, Ian McHarg made the case for a design and planning approach that is sensitive to and compatible with regional environments. McHarg’s basic principle of using nature as a model struck a chord with landscape architects and planners, and it certainly resonated with ecologists.

However, the impact of invasive species—at the hands of well-intentioned designers, gardeners, farmers, and landscapers—has exacerbated a problem that was just beginning to loom when McHarg penned his text.

Many nonnative species are highly invasive, and have compromised the health and stability of natural communities well beyond the point of their introduction.

The spread, success, and dominance of some species is so pervasive that some ecologists have begun to question whether we should merely accept the permanent alteration of natural associations by nonnative species.

These new associations, with various mixes of native and nonnative species, have been called “novel ecosystems.” Do these ecosystems, some of which appear to be stable and self-sustaining, challenge the “design with nature” model?

Or are novel ecosystems different models that might provide important insights for both landscape architects and ecologists?

Novel ecosystems exclude flower gardens and hay fields, for example, but might include vacant lots or mixed-species forests under some management. These increasingly widespread ecosystems (perhaps covering 35 percent of the Earth's land surface) range from near-monocultures of a nonnative species to diverse ecosystems with mixes of native and nonnative species.

Indicators Of Ecosystem Health, As Design Principles

There is little doubt that stability and resiliency, or recuperative potential, are effective ways to evaluate ecosystem health. Healthy ecosystems require the least maintenance while still providing desired services.

The introduction of nonnative plants can create a whole new ecosystem. Photo Courtesy Of Applied Ecological Services, Inc.

Underpinning the stability and the services provided are processes such as primary production, soil development and maintenance, nutrient cycles, pollination, predation, and competition.

These processes are the work of myriad organisms that reside within an ecosystem, whose residence can be described in terms of their composition, distribution, and structure.

Thus, the health or stability of an ecosystem and the services it provides are dependent on the composition, distribution, and structure of the ecosystem.

Generally, healthy ecosystems can be described by five primary characteristics:

Diversity

Productivity

Stinginess

Dynamics

Stability.

Diversity describes all of the species of an ecosystem, including flora, vertebrates, invertebrates, and microorganisms (which are often overlooked) as well as the genetic variability of these organisms, and the variation of community composition over the landscape.

Productivity describes the primary production and net-gross photosynthesis of an ecosystem, as well as the fecundity and mortality of other organisms within the community.

That, in turn, is closely related to soil development and water and nutrient retention, or stinginess .

Dynamics refers to the continual adjustment in populations of species, fluxes in nutrient intake and release, and seasonal changes within a community’s structure as conditions change throughout the year and from year to year.

Stability describes an ecosystem’s ability to adapt to or overcome perturbation with minimal change in other fundamental characteristics. These are the primary modeling criteria for ecosystem restoration or for the design of a sustainable and functional urban landscape.

A focus on these characteristics as design modeling criteria shifts the debate about nonnative species and novel ecosystems from abstract to pragmatic principles. Aesthetics and concerns over human perception, often cited as justification for the need of nonnative species, are completely compatible with characteristics of healthy ecosystems.

These ecosystems are able to adjust to a wide range of environmental perturbations, from pests and disease to variations in climate.

Natural ecosystems are comprised of a diverse suite of species adapted to the local environment that collectively contributes to resiliency. Resiliency is the result of the linkages and redundant functions of component species and their interactions, such as competition, predation, disease, parasitism, and herbivory.

It follows that healthy natural ecosystems, called “reference areas,” are ideal models. The challenge is finding healthy ecosystems in a comparable habitat.

Reference area ecosystems adapt to the local environmental conditions. They are comprised of native species and communities that have adapted for thousands of years in glaciated regions, and longer elsewhere.

Questions about how many generations are required for nonnative species to be sufficiently integrated into natural ecosystems suggest a misunderstanding about the length of time required for delicate balances to develop, and ignore the implied conclusion that these species should integrate with a native system.

This is not to conclude, however, that nonnative species cannot fill niches in a native ecosystem and begin the development of a novel ecosystem. It does suggest, however, that nonnative species will likely alter the ecosystem processes, even if the change is subtle, thereby leading to a different ecosystem.

Impact On Stability And Ecosystem Processes

The horticultural to agricultural continuum is the frontier of nonnative and invasive species because of repeated introduction of species and reduction or elimination of natural ecosystems, thereby providing ample opportunity for the establishment of nonnative species.

Many nonnative species are well adapted to human disturbances and become invasive where environmental resistance to their population expansion is compromised by human development.

Environmental resistance is the sum of adaptations by native species to stressors, such as disease, parasites, predators or herbivores, and competitive relationships with other species.

Ecological stability is achieved when reproduction and environmental resistance are in balance. Interestingly, introduced species often remain quiescent for years, and then populations begin spreading rapidly.

Reasons are unclear. However, in the case of some species, invasion was no doubt exacerbated by massive planting or introduction efforts of humans.

Not all species contribute equally to diversity or ecosystem processes. Indeed, invasive species ultimately result in loss of overall diversity, but not always loss of stability.

Usually, the loss of diversity leads to loss of ecological functions that further impoverish the ecosystem. It is because of nonnative species that natural areas often require active management. A remarkably widespread belief that natural ecosystems will take care of themselves is no longer valid.

Ecosystem restoration design begins by addressing the underlying physical, chemical, and hydrological changes associated with deterioration. Restoration usually involves directly or indirectly mitigating the stressors, then controlling invasive species and reintroducing native species that have been lost or diminished within the community structure.

With restoration, the aim is not necessarily to duplicate past conditions, although by using reference area ecosystems, we gain insight into a community’s composition, structure, and diversity that existed prior to human disturbance.

Instead, the goal is ecological stability and restoration of lost functions or services. As we begin to understand novel ecosystems better, we may gain insight into which nonnative species are invasive, and which are compatible.

Some nonnative species may contribute to desired ecosystem processes, especially if they are better suited to stressors that cannot be corrected.

Where nonnative plants are given the opportunity to grow, prosper, and spread, they may have both positive and negative effects on other species, as well as on ecosystem processes.

Thus, it is necessary to consider each species and each situation independently. Kudzu, for example, is effective in preventing erosion, intercepting precipitation, and providing screening against noise. It is a legume with nitrogen-fixing capability, produces beautiful, fragrant flowers that can attract some native pollinators, and is edible.

However, kudzu is aggressively invasive and leads to a dramatic reduction of all other plants.

Nonnative species especially may be useful where no native species are adapted to the highly altered conditions, but it should be a matter of ethics that new species are screened for invasive potential before being used.

Summary

Designing with nature requires knowledge that increases with experience, often more quickly than one might expect.

When designers and ecologists collaborate, the durability of design is improved, and the experiences of users are enhanced, while the designs of novel ecosystems in isolation have fallen short.

The debate over native versus nonnative species is largely moot when the models of appropriate ecosystem services are derived from reference ecosystems. Within the ecosystem-characteristics framework proposed above, locally adapted native species provide comparable or often superior performance than nonnative counterparts in a novel ecosystem, usually at lower cost.

Native species are less risky. This applies in both large and small areas. In urban lands and pocket parks, native species not only provide the cultural landscape desired but also support rare pollinator insects, a range of beneficial soil organisms, and even birds or larger organisms not common in the urban fabric.

If these are not reasons enough, now more than ever the marketplace encourages the use of native plants and discourages the use of nonnative species.

The indirect cost of invasive species, estimated at hundreds of billions of dollars annually, can no longer be ignored.

The framework proposed above is a refinement of McHarg’s charge; it does not alter his premise but provides greater detail for landscape architects and planners when designing and managing ecosystems.

For the land designer and the land steward, this refinement opens even more common ground to explore with practicing and experienced restoration ecologists.

Steven I. Apfelbaum is a Senior Ecologist certified with the Ecological Society of America, and Chairman of Applied Ecological Services, which works worldwide on the design and construction of urban landscapes and ecological restoration projects.

Alan Haney is emeritus professor of forest ecology and former dean of the College of Natural Resources, University of Wisconsin-Stevens Point. Haney has been recognized repeatedly as a distinguished educator. He is author of several books on ecosystem restoration and management, and a widely published scientist.

Jacob Blue , RLA, is a senior landscape architect and Design Director at Applied Ecological Services, Inc. He provides design direction and oversight as well as experiential expertise for large- and small-scale design and restoration projects. Because of his understanding of native plants and his design strengths, Blue is keenly interested in the use of native species in both restoration and non-restoration designs as well as the habitat and aesthetic implications of their use.

Sidebar #1

Characteristics of healthy ecosystems and adaptive design principles:

Dynamic and responsive . As conditions change, both the genetic makeup and populations of species respond, leading to domino effects in relationships between species.

Resilient . Following a disturbance or sudden change in environmental conditions, individual species respond with both physiological and population-level adjustments, and with corresponding changes in interactions of populations … ultimately resulting in recovery of steady-state conditions. This process—affecting all aspects of structure, composition, and functions in the ecosystem—is called succession.

Redundant . Many species have overlapping niches and perform similar functions, which contribute to efficiency, resiliency, and stability.

Self-organized . Community associations reflect long-standing evolutionary relationships with each species and each association located in the landscape where it is best adapted to compete, and with remarkably similar associations in similar habitats.

Self-replicating . When disturbed, communities tend to return to the pre-disturbance structure and composition, and similar associations will develop in similar habitats if no barriers exclude key species.

Conservative. All waste is recycled, and nutrients are largely retained. Any water leaving the system is relatively free of contaminants.

Having a hierarchy of temporal and spatial scales . Small scales of organization and functions contribute to larger scales of organization and functions.

Continuous and open. Ecosystems exist along spatial and temporal continua, such that sharp edges are rare, although some gradients may be steep.

Functional. Some functions cease or decline at small scales, such as minimal “home range” required by some species, or minimal time required for meaningful evolution or soil development.

Moving along gradients. Transitions from one association or habitat to another, called ecotones, is where many functions are more common, for example, hybridization and evolution. Movement of species along gradients is also one of the important ways ecosystems remain dynamic and responsive.

Vulnerable to catastrophic disruption . Disturbances that exceed the evolutionary experience of species or communities can lead to collapse of resiliency.

Sidebar #2

Design principles:

Recognize and retain, or restore/create community ecotones with supporting environmental gradients.

Use locally sourced native plants from the same physiographic province, or closer if the project interacts with high-quality natural areas or scientific areas.

Manage stormwater retention, infiltration, and interception within the landscape to emulate hydrographs of matched healthy ecosystems.

Do not introduce nonnative or native invasive species.

Allow designs to be dynamic, and foster species and community shifts as conditions change.

Create a design that results in low contaminant concentration (including nutrients) and suspended solids in runoff or infiltrating waters.

Design plans that build and protect soil.

Develop communities that emulate structure, composition, and patterns found in local native-plant communities.

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